The invention relates to a method for thermally drying wet-ground cement raw meal in the form of sludge for the purpose of manufacturing a dry meal, which is introduced into the cyclone suspension type heat exchanger system of a cement clinker production line working according to the dry method, with a dryer with utilization of the heat contained in the gaseous flows of the of the cyclone suspension type heat exchanger system and with separation of the dry meal from the exhaust vapors formed during drying.
In the so-called wet method for production of cement clinkers raw materials are wet-ground in tube mills in order to obtain in this manner a fine grained and intimately mixed raw sludge. This raw sludge is introduced in doses into a very long rotary kiln, dried there and burned into cement clinkers. It goes without saying that during drying the complete evaporation heat contributes to the loss of gaseous flows in such wet rotary kilns.
In the course of time the more favorable thermal dry method has asserted itself, in which the dry ground raw materials are burned into cement clinkers in a comparatively short rotary kiln with a serially connected cyclone suspension type heat exchanger system.
In the conversion of existing wet plants into modern dry plants it is known to retain the wet preparation of the raw material and to convert the raw material in the form of sludge, i.e. the raw meal sludge by means of a mechanical and subsequently thermal dehydration in the degree of dryness required for loading into the cyclone suspension type heat exchanger. For example, it is known from DE-A-43 40 382 to mechanically dehydrate a raw meal sludge in a filter press, to thermally dry the filter cake in a dryer system with utilization of the heat of the cement clinker production to calcine it in a calcining step and to burn it into cement clinkers in a rotary kiln, said clinkers being cooled in a clinker cooler. The dryer system used in the process is a so-called flow dryer, comprising an impact hammer crusher permeated by hot gas, a riser, a separator with recirculation of coarse grain to the impact hammer crusher and a separator e.g. filter for separation of the dried raw meal from the drying gas.
Disadvantageous in this known production of cement clinkers is the driven technical expenditure for the mechanical dehydration and drying of the filter cake, in particular the use of the impact hammer crusher or drying crusher with rotating machine parts. For example the impact hammer crusher of the known flow dryer must be dimensioned in its size and performance in such a way that the hammers can also be passed through the still damp material on the impact hammer crusher bottom in the case of an insufficient rate of drying. In addition to this comes the comparatively high wear of the impact hammer crusher or rotary dryer caused by the parts rotating at a high speed. In the case of the known flow dryer system it cannot be avoided that the greater part of the water vapor heat content is to be added to the loss of gaseous flows. For this reason the conversion of an existing wet system for the production of cement clinkers to a dry method can be uneconomical, if one considers the fact that the specific heat energy requirement of such a converted cement clinker production line still amounts to approximately 900 kcal/kg clinkers.
A fluid bed dryer using vapor for the drying of a clear sludge containing 96% water is known from U.S. Pat. No. 3,654,705. However, a fluid bed dryer cannot be operated with such fine grained solids of a fineness such as cement raw meal must exhibit, namely a fineness of 85%<90 μm, which corresponds to normal cement raw meal fineness.